Would you want to live on Gliese 581C?

The recent discovery of a terrestrial like planet (i.e., one which is stony and not gaseous like Jupiter) gave rise to a minor media hype, with statements like "We may be witnessing the start of a cosmic real estate boom" (e.g., here) being commonplace. However, is Gliese 581C a place you would like to call home at any point in the future?

The planet, which is likely to have a mass five times larger than our own's, will have a surface gravity which is about 51/3~1.7 times larger (assuming a similar density). That is, you'll be awfully tired. During my military service, I once flew on a mission where the plane would perform a U-turn every 15 minutes or so. After a few turns, I realized that there is no point trying to keep on standing at the higher effective gravity. I cannot imagine spending 24hrs a day at this gravity.

In fact, talking about days, there are probably none on Gliese 581C. Why? Because Gliese 581 is very close to its parent star. This implies that the tidal interaction is very large. Typically, it scales like the mass of the main object and the distance to it, to the 3rd power. This implies a few things:

If the planet did not tidally lock, one would expect very high tides. Since the tidal forces are typically 0.3*143~800 times larger, instead of the few decimeter-high tides experienced in the Earth's oceans, Gliese would typically have 100 meter tides. This would dwarf the 16m tides at the bay of fundy!

The large tidal interaction would actually slow down the spin of the planet on a time scale way shorter than on Earth. Since this time scale typically scales as the tidal forces squared, the slowing down rate would typically be a million times shorter. i.e, VERY FAST. That is, as soon as the oceans formed (optimistically assuming Gliese 581 did form oceans like those on Earth), Gliese 581C stood still (well, tidally locked to its orbit).

Of course, if Gliese did not form Oceans, and its surface is rock solid, the tides would be much smaller. This would allow it to be free of tidal locking for much longer, but the lack of Oceans would imply that it cannot have any romanic beaches to brag about.

Standing on a shore some 80° from the permanent point facing the star. Here, the huge disk of the M-dwarf star permanently appears to be just about ready to set. The red colors are natural to the cool red star, they're not an atmospheric effect. Unlike the wet rainy tropical sun facing point, here the cool cyclon-less climate makes it permanently pleasant, dry though annoyingly red.

A tidally locked planet implies that at any given location, the main star always appears at the same point in the sky, which is how you would see Earth if you happened to be on the Moon. The difference is in the size. The star would appear about 7 times larger than the sun appears to us, or twice as a large as Earth appears from the moon.

Because of the tidal locking, the star pointing point would be very hot, the equators cool, and the permanently dark side very cold! Unlike Earth, which has the Hadley circulation advect energy polewards up to 30°, and beyond that with cyclones, tidally locked Gliese 581 could have convective cells which extend the whole sun facing side and beyond, but no cyclonic activity.

The large apparent diameter of the star would allow you to clearly see the real problem with living close to an M-dwarf star. You could easily see the HUGE flares the such stars exhibit - many times larger than those of the sun.
Their origin lays in the fact that while only 2% of the sun's mass is transporting energy through convection (e.g., the movement seen in a heated pot of water or soup), most of the mass in M-dwarfs is transporting energy convectively. Since the convective layer is responsible for the generation of the solar magnetic field and the non-thermal solar activity (UV, X-rays, sunspots, magnetic fields, solar wind and of course solar flares), all these effects would be way larger in Gliese's M-dwarf star. Thus, large flares, which are orders of magnitude larger than those seen on the Sun would likey be very harmful to us, if we ever tried to settle on Gliese 581 C. (But not necessarily to the ocean dwelling beings which did form there ;-) )

The last point to note is that after a while, the fact that everything appears deep red would really annoy any sane person...

There is no place like the planet we call home. Well, at least Gliese 581C is no competition.

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Comments (3)

Martin (not verified)

I read in the newspapers that the average temperature of this planet should be between 0C and 40C.

Where does this range come from? Is it random numbers only created to capture the attention of the public? What about the atmospheric composition of this planet? It doesn't matter to establish such temperature range?

According to the mass media, the massive CO2 atmosphere of Venus is the main reason for its extreme temperature of around 450C. So, if this planet has a Venus-like atmosphere, does the 0-40C range remain valid (if it is "valid" at the first place...)?

May 05, 2007

shaviv

The average temperature of 0 to 40°C, is given from a radiative equilibrium between the radiation which this planet will receive at its distance and the radiation it will emit, assuming a black body. A similar calculation for earth would give you -5°C or so. As we know well, the average temperature near the surface is 20°C higher. This is because of the greenhouse warming of water vapor (+30°) and the reflection from clouds (-10° or so).

Since we don't know the albedo of the planet (how much light it reflects) or what is it the optical depth of the atmosphere to IR radiation, we don't know the surface temperature on the planet.

The Gaia hypothesis states that once life forms evolution will direct life to
alter its environment to remain suitable for life. Gliese C is in the right size
and temperature range for water based life to form. Hopefully future observation will be able to get a spectrum for the atomosphere, which should give us insight into weather theres life there.